1. Field of the Invention
The present invention relates to machinery structural parts which are formed by a cold working and are to be produced by strengthening through an induction quenching, the parts being required to have high rolling fatigue strength and torsion fatigue strength, for example, in an outer race for a joint of constant velocity, and a steel for induction quenching to be used thereto.
2. Description of the Related Art
In general, as machinery structural parts required to have the high fatigue strength such as the outer race for joints of constant velocity, medium carbon steels containing C: 0.40 to 0.60% are used. These steels are formed through a cold forging, and then increased in surface hardness by the induction quenching treatment so as to enhance the rolling fatigue strength and the torsion fatigue strength. These machinery structural parts have recently been demanded for further improving higher strengthening and cold workability because of making light weight.
Since the medium carbon steel is generally poor in the cold workability, many techniques for improving it have been developed. For example, JP-A-62-23929 and JP-A-62-196327 disclose technologies that Si and Mn in steel are limited, deoxidation and denitrification are carried out by Al and Ti, a fine amount of B is added to guarantee a high hardenability with the amount of small alloying addition, and temperature conditions of hot rolling or finish rolling temperature are controlled for improving the cold workability.
JP-A-2-129341 discloses a method for improving the cold workability of steel by limiting amounts of Si and Mn, decreasing alloying elements by adding Al, Ti and B as the above two examples, and limiting upper limits of N, S and O.
On the other hand, enhancing of strength, in particular improvement of fatigue strength mainly depend upon hardening in a skin portion by the induction quenching and compressive residual stress generated thereby, and efforts are directed to adjusting of chemical compositions in steel for efficiently demonstrating effects by the induction quenching. In parts requiring the rolling fatigue strength as outer races for joints of constant velocity, it is desirable that the hardness of the rolling face is high, but if the hardness is enhanceed, notch sensibility is increased resulting to invite a lowering of the fatigue strength, and so the enhancing of hardness is limited.
It is known that, in a hard steel, non-metallic inclusions in steel serve as sources of stress concentration and lowers the fatigue strength of steel. JP-A-2-129341 discloses a method of limiting an upper limit of O content to 0.0020%, taking prevention of deterioration of rolling fatigue life into consideration, and limiting an upper limit of Ti content to 0.05%, paying attention to prevention of forming large nitrides harmful to the rolling fatigue life.
The fatigue strength may be enhanced to a certain extent by providing methods of adjusting chemical compositions in steel as mentioned above, however, it has been difficult to decrease dispersions of the fatigue strength, in particular dispersions of the rolling fatigue life.